1. Field of the Invention
The present invention relates to a light source driver, light source-driving method, and image-forming apparatus. More specifically, the present invention relates to a light source driver and light source-driving method which drive a light source based on lighting information, and an image-forming apparatus including the light source driver.
The present invention also relates to a light source-driving circuit which drives a light source and an optical scanner.
2. Description of the Related Art
An image-forming apparatus such as an optical printer, digital copier, and optical plotter is configured to drive a light source based on a signal pulse-modulated according to image information.
For example, Patent Document 1 (JP 2009-229762A) discloses a method of correcting light volume including a step of converting a pixel value of image data into an output gradation value, a step of detecting falling when the output graduation value of the dot in the downstream side in the main scanning direction is smaller than the output gradation value of the dot in the upstream side by a predetermined value or more, a correction step of adding an undershoot correction amount to the output gradation value of the dot just after detecting the falling, and a step of controlling the volume of the laser beam according to the output gradation value.
Patent Document 2 (JP 4476568B) discloses a light source driver including a generator which generates a superimposed current approximately corresponding to a charge-discharge current to a capacity generating in parallel with a light source in a predetermined time near at least one of the rising and falling of the driving current of the light source, an adder-subtractor which adds or subtracts the superimposed current generated by the generator to the driving current, and a controller which controls a superimposed time for generating a superimposed current according to a capacity, the controller controlling the superimposed time according to a variation in the driving current.
Patent Document 3 (JP 2011-198877A) discloses a semiconductor laser driver for setting a correction current value which determines a property of rising of an output and/or a property of falling of an output according to a value of current for driving a semiconductor laser.
The demand for an improved image quality in an image-forming apparatus increases every year. In the image-forming apparatus using the method and the driver disclosed in Patent Documents 1-3, optical waveforms may vary, and it is difficult to obtain an image quality of a required level.
In an image-forming apparatus for use in conventional product printing or the like, a photoreceptor is exposed by obtaining a predetermined light output from a light source such as an LD, so as to express a concentration of an image.
It is known to generate an emission delay time depending on a response property of a light source before obtaining a predetermined light output from a light source. It is also known to generate an emission delay time depending on a parasitic capacity such as a circuit on which a light source is mounted before detecting a light output after supplying a driving current to a light source, for example.
For this reason, a response property of a light output is deteriorated due to the emission delay time in the conventional image-forming apparatus. When, for example, the light output time is set to a short time of several nsec or below, the light output becomes smaller than a predetermined light volume, so that an uneven image may be generated due to a decrease in a concentration of an image.
Various measures are conventionally adopted for improving a response property of a light output. For example, Patent Document 4 (JP H04-146546A) discloses, as a conventional technique regarding light source-driving control using an LD, applying an overshoot current in light volume increase timing and an undershoot current in light volume decrease timing when changing a light volume of a lead level and a light level.
Patent Document 5 (JP H02-215239A) discloses to speed up discharge and charge of electric charge accumulated in a parasitic capacity of an emission element by an overshoot current or an undershoot current for an optical response.
The undershoot current in the conventional technique is a current for improving rounding of falling of the optical output waveform, but its value is a predetermined fixed value. When the light volume emitted from the emission element, for example, is changed, the undershoot current does not become an appropriate value, so that a response property of a light output may not be significantly improved.